1. Field of the Invention
The present invention relates to an atomic absorption spectrophotometer which performs quantitative analysis of metal elements contained in various substances, and more particularly, to a furnace-type atomic absorption spectrophotometer.
2. Description of the Related Art
In an atomic absorption spectrophotometer, light originating from a holocathode lamp which serves as a light source is decomposed into spectra by a spectrometer, and the spectra reach a detector. A detector signal voltage is dependent on the type of an element of the holocathode lamp, a characteristic of the spectrometer, and a spectral sensitivity characteristic of the detector.
In the furnace-type atomic absorption spectrophotometer, a sample is held in a graphite tube (heating tube), and the tube is heated, whereby the sample is heated to a high temperature and eventually atomized. A measuring beam is caused to pass through atomic vapors, to thus measure absorbancy. Therefore, when the graphite tube has reached the maximum temperature; that is, when the sample is atomized, the graphite tube radiates light. The thus-emitted light also reaches the detector.
The detection signal including the emitted light may saturate an analog circuit or may exceed the maximum voltage which can be converted by an analog-to-digital converter, because the detection signal is excessively strong. However, when the detection signal has exceeded the maximum voltage, accurate measurement of the signal cannot be carried out. For this reason, an optimum detection signal voltage is set by controlling an amplification factor of the detection signal before measurement (see, e.g., JP-A-2002-71558).
This detection signal voltage is usually set to 50 to 70% of the maximum voltage which can be measured within the atomic absorption spectrophotometer. In consideration of noise of the analog circuit or quantization noise of the analog-to-digital converter, both being employed in the atomic absorption spectrophotometer, a better signal-to-noise ratio is obtained as the voltage of the detection signal approaches 100%. However, in order to prevent saturation of the voltage of the detection signal, which would otherwise be caused by the light emitted from the graphite tube, the voltage is held down to 50 to 70% of the maximum voltage.
Under the related-art method, only a given margin for the light emitted from the graphite tube is expected within an ordinary range of measurement requirements, and no allowance is given for variations in the intensity of emitted light which would arise depending on measurement requirements. The longer the wavelength, the wider the slit width, and the higher the atomizing temperature, the greater the intensity of the light emitted from the graphite tube. Therefore, the voltage of the detection signal may exceed the maximum measurable voltage, depending on the wavelength, the slit width, and the atomizing temperature.
In such a state, the voltage has been adjusted by the operator manually rendering the slit width narrow. However, when the slit width has been made narrow, the quantity of light entering the detector is decreased, whereby the signal-to-noise ratio is deteriorated.
According to another method, an amplification factor is changed during the course of atomizing operation in response to the detector signal. However, atomization arises during a period of one second or thereabouts, thus raising demand for a circuit which is highly responsive such that it can set the amplification factor in response to the atomization. Further, the amplification factor to be set also requires accuracy, and hence such a circuit becomes expensive.